716 research outputs found
Comment on Ricci Collineations for type B warped space-times
We present two counterexamples to the paper by Carot et al. in Gen. Rel.
Grav. 1997, 29, 1223 and show that the results obtained are correct but not
general.Comment: LaTex, 3 pages, Eq. (9) and reference added, typos corrected; Gen.
Rel. Grav (to appear
Source-Channel Diversity for Parallel Channels
We consider transmitting a source across a pair of independent, non-ergodic
channels with random states (e.g., slow fading channels) so as to minimize the
average distortion. The general problem is unsolved. Hence, we focus on
comparing two commonly used source and channel encoding systems which
correspond to exploiting diversity either at the physical layer through
parallel channel coding or at the application layer through multiple
description source coding.
For on-off channel models, source coding diversity offers better performance.
For channels with a continuous range of reception quality, we show the reverse
is true. Specifically, we introduce a new figure of merit called the distortion
exponent which measures how fast the average distortion decays with SNR. For
continuous-state models such as additive white Gaussian noise channels with
multiplicative Rayleigh fading, optimal channel coding diversity at the
physical layer is more efficient than source coding diversity at the
application layer in that the former achieves a better distortion exponent.
Finally, we consider a third decoding architecture: multiple description
encoding with a joint source-channel decoding. We show that this architecture
achieves the same distortion exponent as systems with optimal channel coding
diversity for continuous-state channels, and maintains the the advantages of
multiple description systems for on-off channels. Thus, the multiple
description system with joint decoding achieves the best performance, from
among the three architectures considered, on both continuous-state and on-off
channels.Comment: 48 pages, 14 figure
Self-similar Bianchi models: II. Class B models
In a companion article (referred hearafter as paper I) a detailed study of
the simply transitive Spatially Homogeneous (SH) models of class A concerning
the existence of a simply transitive similarity group has been given. The
present work (paper II) continues and completes the above study by considering
the remaining set of class B models. Following the procedure of paper I we find
all SH models of class B subjected only to the minimal geometric assumption to
admit a proper Homothetic Vector Field (HVF). The physical implications of the
obtained geometric results are studied by specialising our considerations to
the case of vacuum and law perfect fluid models. As a result we
regain all the known exact solutions regarding vacuum and non-tilted perfect
fluid models. In the case of tilted fluids we find the \emph{general
}self-similar solution for the exceptional type VI model and we
identify it as equilibrium point in the corresponding dynamical state space. It
is found that this \emph{new} exact solution belongs to the subclass of models
, is defined for and
although has a five dimensional stable manifold there exist always two unstable
modes in the restricted state space. Furthermore the analysis of the remaining
types, guarantees that tilted perfect fluid models of types III, IV, V and
VII cannot admit a proper HVF strongly suggesting that these models either
may not be asymptotically self-similar (type V) or may be extreme tilted at
late times. Finally for each Bianchi type, we give the extreme tilted
equilibrium points of their state space.Comment: Latex, 15 pages, no figures; to appear in Classical Quantum Gravity
(uses iopart style/class files); (v2) minor corrections to match published
versio
Note on Matter Collineations in Kantowski-Sachs, Bianchi Types I and III Spacetimes
We show that the classification of Kantowski-Sachs, Bianchi Types I and III
spacetimes admitting Matter Collineations (MCs) presented in a recent paper by
Camci et al. [Camci, U., and Sharif, M. {Matter Collineations in
Kantowski-Sachs, Bianchi Types I and III Spacetimes}, 2003 Gen. Relativ. Grav.
vol. 35, 97-109] is incomplete. Furthermore for these spacetimes and when the
Einstein tensor is non-degenerate, we give the complete Lie Algebra of MCs and
the algebraic constraints on the spatial components of the Einstein tensor.Comment: 10 pages, Latex. Accepted for publication in General Relativity and
Gravitatio
Modified brane cosmologies with induced gravity, arbitrary matter content and a Gauss-Bonnet term in the bulk
We extend the covariant analysis of the brane cosmological evolution in order
to take into account, apart from a general matter content and an
induced-gravity term on the brane, a Gauss-Bonnet term in the bulk. The
gravitational effect of the bulk matter on the brane evolution can be described
in terms of the total bulk mass as measured by a bulk observer at the location
of the brane. This mass appears in the effective Friedmann equation through a
term characterized as generalized dark radiation that induces mirage effects in
the evolution. We discuss the normal and self-accelerating branches of the
combined system. We also derive the Raychaudhuri equation that can be used in
order to determine if the cosmological evolution is accelerating.Comment: 12 pages, no figures, RevTex 4.0; (v2) new references are added;
(v3,v4) minor changes, acknowledgment is included; to appear in Phys. Rev.
A geometric description of the intermediate behaviour for spatially homogeneous models
A new approach is suggested for the study of geometric symmetries in general
relativity, leading to an invariant characterization of the evolutionary
behaviour for a class of Spatially Homogeneous (SH) vacuum and orthogonal
law perfect fluid models. Exploiting the 1+3 orthonormal frame
formalism, we express the kinematical quantities of a generic symmetry using
expansion-normalized variables. In this way, a specific symmetry assumption
lead to geometric constraints that are combined with the associated
integrability conditions, coming from the existence of the symmetry and the
induced expansion-normalized form of the Einstein's Field Equations (EFE), to
give a close set of compatibility equations. By specializing to the case of a
\emph{Kinematic Conformal Symmetry} (KCS), which is regarded as the direct
generalization of the concept of self-similarity, we give the complete set of
consistency equations for the whole SH dynamical state space. An interesting
aspect of the analysis of the consistency equations is that, \emph{at least}
for class A models which are Locally Rotationally Symmetric or lying within the
invariant subset satisfying , a proper KCS \emph{always
exists} and reduces to a self-similarity of the first or second kind at the
asymptotic regimes, providing a way for the ``geometrization'' of the
intermediate epoch of SH models.Comment: Latex, 15 pages, no figures (uses iopart style/class files); added
one reference and minor corrections; (v3) improved and extended discussion;
minor corrections and several new references are added; to appear in Class.
Quantum Gra
Governance implications of the UN Higher Education Sustainability Initiative
The Higher Education Sustainability Initiative (HESI, 2017a) states that higher education institutions (HEIs) are integrating the SDGs into sustainability strategies in the form of research, teaching, pedagogy, and campus practices, and to position HEIs as key drivers for achieving the UN Sustainable Development Goals (SDGs). Concern has been raised (HESI, 2017b) as to the potential impact of HEIs in helping to achieve the SDGs; the challenges faced by HEIs with integrating the SDGs into curriculum and institutional strategies; the role of partnerships for HEIs among students, faculty, government, and various stakeholders; and how the adoption of the 2030 Agenda for Sustainable Development, including the SDGs, will transform the work of HEIs. Prior research has highlighted the importance of interdisciplinary and trans-disciplinary research and studies (e.g. Mader & Rammel, 2015); and more recently Rasche et al (2017) conceive that governance systems in general can serve to make, take, or break support for the SDGs. In fact, Velazquez et al., (2005) found many obstacles preventing the success of sustainability initiatives on campuses around the world. This paper reviews progress of a sample of 300+ HEIs that are signatories to the HESI. Findings reveal a difference between HEI governance that is ‘instrumental’ and governance that is ‘holistic’ in relation to sustainability with implications for achieving the SDGs in general and for academic-business partnerships in particular. The research is supported by a grant from Enterprise Educators UK (EEUK, 2017) a network of 1,600 enterprise and entrepreneurship educators and practitioners from over 100 UK Higher and Further Education Institutions and related organisations
Self-similar Bianchi models: I. Class A models
We present a study of Bianchi class A tilted cosmological models admitting a
proper homothetic vector field together with the restrictions, both at the
geometrical and dynamical level, imposed by the existence of the simply
transitive similarity group. The general solution of the symmetry equations and
the form of the homothetic vector field are given in terms of a set of
arbitrary integration constants. We apply the geometrical results for tilted
perfect fluids sources and give the general Bianchi II self-similar solution
and the form of the similarity vector field. In addition we show that
self-similar perfect fluid Bianchi VII models and irrotational Bianchi
VI models do not exist.Comment: 14 pages, Latex; to appear in Classical and Quantum Gravit
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